An optical network generally comprises a plurality of stations able to send and receive optical signals to and from other stations of the network. Information is exchanged by means of optical links to which access nodes serving respective stations are connected.
The wavelength-division multiplexing (WDM) technique is advantageously used to exploit the bandwidth capacity of these optical links. Accordingly, the optical links convey multiplex signals that are formed from a combination of optical signals each consisting of an optical carrier wave modulated as a function of the information to be sent. Each carrier wave has a specific wavelength that defines a corresponding channel.
Moreover, if the network is sufficiently small, devices for regenerating the channels individually may be dispensed with. A network of this kind is referred to as “transparent” and may nevertheless comprise optical amplifiers to amplify simultaneously the channels of the transmitted WDM multiplexes.
The invention applies to a type of transparent WDM network that uses a ring configuration. The network then comprises an optical link which has one end coupled to a send interface of a concentrator (also known as a “hub”) and its other end coupled to a receive interface of the same concentrator. The concentrator normally also communicates with an external interconnection network.
FIG. 1 is a diagram that represents one example of a network of the simplest kind. In this example, the looped link consists of a fiber F to which access nodes AN1-AN3 for send terminals TX and receive terminals RX of associated stations ST1-ST3 are connected. The optical link is made up of a plurality of fiber segments FS1-FS4 separated by nodes and possibly by optical amplifiers (not shown). The link has a first end, called the upstream end, connected to the send interface HT of the concentrator HUB and a second end, called the downstream end, connected to the receive interface HR of the concentrator.
The send interface HT comprises a plurality of senders using carrier waves at different send wavelengths. Each station also includes a wavelength-selective receiver. Accordingly, each sender of the concentrator can inject a signal of given wavelength into the link, and when that signal reaches an access node via the fiber it can be acted on by the associated station if one of its receivers is tuned to that wavelength.
Conversely, the receive interface HR of the concentrator comprises a plurality of different wavelength-selective receivers and each station includes a sender TX that sends at a given send wavelength. Accordingly, a sender of a station can inject a signal of given wavelength into the link and when that signal reaches the receive interface it can be acted on by the concentrator if one of its receivers is tuned to that wavelength. However, to prevent interference and conflicts at the level of the receive interface, it is necessary for all the send wavelengths of the various stations to be different from each other.
It should be noted that, given the bandwidth of the link, each station could in practice-comprise a plurality of receivers and a plurality of senders tuned to a plurality of wavelengths.
Accordingly, signals sent by the concentrator and carried by respective wavelengths are routed to destination stations having respective receivers tuned to those wavelengths.
The signals coming from the concentrator then constitute what is referred to as “downlink” information traffic on the link.
Information is exchanged between the stations via the concentrator in the following manner. Each send station sends the concentrator a signal that is carried by one of the receive wavelengths of the concentrator and contains an address indicating the destination station. After reception of the signal and its conversion to electrical form by the concentrator, the network management means use the destination address to determine a receive wavelength of the destination station. The signal is then converted back into optical form on a carrier wave having that wavelength.
The signals coming from the stations then constitute “uplink” information traffic on the link. It should be noted that the uplink and downlink traffic signals are in fact conveyed in the same “upstream to downstream” direction, i.e. in the direction of propagation of the signals from the senders of the concentrator or the stations to the downstream end of the fiber.
Add-drop multiplexers are routinely used to implement access nodes. Their function is to drop from a received multiplex the channels that have the reserved receive wavelengths of the station concerned.
FIG. 2 is a diagram of one embodiment of this kind of add-drop multiplexer for an access node ANi of a station STi (with i=1 to 3).
It includes a demultiplexer DEMUX having one input and multiple outputs and a multiplexer MUX having multiple inputs and one output. The outputs of the demultiplexer are tuned to non-adjacent wavelength bands and the inputs of the multiplexer are tuned to the same wavelength bands, respectively. The combination of these bands cover the whole of the spectrum assigned to the channels of the network. One of the outputs of the demultiplexer is routed to the receivers RX of the station concerned and is therefore used to drop channels whose wavelengths are reserved for reception by the station. Similarly, one of the inputs of the multiplexer is connected to the senders TX of the station concerned and is therefore used to add channels whose wavelengths are reserved for sending by the station. The other outputs of the demultiplexer are coupled to respective homologous inputs of the multiplexer.
As indicated above, a receiver RX is in fact a plurality of wavelength-selective receivers (which are not shown individually in the figure) operating in the band of wavelengths of the drop output of the demultiplexer. Wavelength-selective reception is effected by means of photodetectors preceded by respective appropriate filters, for example.
This system ANi is inserted between two successive fiber segments FSi, FSi+1 to receive a multiplex that represents the whole of the traffic reaching the upstream side of the node concerned and to send in the downstream direction a multiplex that is formed of signals of channels that are not dropped and signals sent by the station.
Apart from the fact that this solution based on a multiplexer and a demultiplexer is relatively costly, in particular if there are many bands, it has the drawback of limiting flexibility in respect of the number and choice of send and receive wavelengths that can be assigned to the various stations. This solution implies reserving for each station fixed and identical spectral resources both for dropping and for adding. It may in practice be necessary to provide sending capacities for certain stations that are very different from their receiving capacities. Moreover, good flexibility in the choice of wavelengths allows better adaptation to the changing distribution of traffic between the nodes and the concentrator.
Finally, the access nodes as described above are not well suited to easy and economic evolution of the send and receive capacities of the stations as and when requirements increase. To increase the number of users connected to the nodes, it is necessary to add senders and receivers in the stations. It is therefore also necessary to intervene at link level to replace the add-drop multiplexers by others of greater capacity or to provide in the installation devices that are designed to support a high nominal capacity that is initially greater than is really necessary.
There exist other types of add-drop multiplexers based on rejection filters that may or may not be reconfigurable, such as that described in U.S. Pat. No. 6,038,045, for example. These solutions have analogous limitations from the points of view of flexibility in the choice and number of wavelengths.
To overcome these limitations, it is generally necessary for the node to allow a connection between the link and the associated station that is not wavelength-selective, both for dropping multiplex signals received from the upstream side and for adding signals created in the station to the downstream multiplex. In other words, it is necessary to use simple couplers to sample, without filtering, a portion of the optical power received from the link, and to inject the combination of the send signals of the station into the link without filtering them.
FIG. 3 shows an embodiment conforming to the above principle. Here the node ANi consists simply of a 2-to-2 coupler C. A first input of the coupler is connected to the upstream segment FSi and a first output of the coupler is connected to the downstream segment FSi+1.
The station STi includes a send system consisting of n senders TX (n is equal to 4 in the example shown) whose outputs are connected to corresponding inputs of an n-to-1 coupler K (a 4-to-1 coupler in this example). The output of the coupler K is coupled to the second input of the coupler C. The station STi also includes a receive system comprising a demultiplexer DM having an input coupled to the second output of the coupler C and m outputs coupled to m photodetectors RX (there are four photodetectors in the example shown) via an m x m space switch SW (a 4×4 switch in this example).
Accordingly, the two adjacent segments FSi, FSi+1 are connected to each other by a first channel of the coupler C, the upstream segment FSi is connected to the receive system by a second channel and the send system is connected to the downstream segment FSi+1 by a third channel, these three channels providing connections that are not wavelength selective.
Consequently, by providing wavelength-tunable senders TX, each sender can add into the downstream segment a signal carried by any wavelength and there is no limit on the number of senders.
Similarly, by means of the switch SW, each photodetector can receive any of the channels dropped by the demultiplexer DM. The effective receive capacity is determined by the number of photodetectors used, within an overall limit set by the size of the demultiplexer DM, and can evolve easily by adding photodectors and/or installing a larger demultiplexer DM, without disturbing traffic in transit via the node. Note that a plurality of stations can process the same channel, to allow broadcasting of the send signal to a plurality of receivers in different stations.
That solution provides great flexibility in choosing send and receive wavelengths. That choice is nevertheless limited by several conditions.
As is the case in the preceding solutions, the send wavelengths used simultaneously by all of the stations must all be different, of course.
Any station send wavelength must also be different from any concentrator send wavelength, a condition that leads to defining within the set of wavelengths usable in the network (for example band C of the ITU standard) two non-adjacent subsets of wavelengths respectively grouping wavelengths reserved to the senders of the concentrator and wavelengths reserved to the senders of the stations, i.e. to subsets of channel wavelengths respectively assigned to downlink and uplink traffic.
Now, if the evolution of the use of channels along the link, starting from its upstream end (at the output of the send interface of the concentrator), is statistically analyzed, it is found that the number of wavelengths of the concentrator that continue to be useful decreases on moving from node to node in the downstream direction, i.e. on approaching the receive interface of a concentrator.
This means that the set of spectral resources of the network is not used optimally.